1. Department of Earth Resource Science, Graduate school of International Resource Sciences, Akita University, Tegata-Gakuenmachi 1-1, Akitashi, Akita 010-8502, Japan (e-mail: )
2. Department of Geology and Paleontology, National Museum of Nature and Science, Amakubo 4-1-1, Tsukuba-shi, Ibaraki 305-0005, Japan
3. Center for Advanced Marine Core Research (CMCR), Kochi University, Monobe-Otsu 200, Nankoku-shi, Kochi 783-8502, Japan
4. Climate Geochemistry, Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany